Machine-Type Communication (MTC) is a form of data communication involving one or more entities that do not necessarily need human interaction. MTC is an important and growing revenue stream for wireless network operators. MTC devices, such as monitors, sensors, controls, etc., may also be referred to as MTC user equipment (UE). Operators benefit from serving MTC devices with already deployed radio access technology. For example, long term evolution (LTE) is a competitive radio access technology for efficient support of MTC.
The Third Generation Partnership Project (3GPP) LTE specification defines battery life, device cost/complexity, and coverage optimizations for MTC. MTC devices are sometimes located in challenging locations, for which LTE network rollouts were not dimensioned for full coverage. For example, smart meters are often placed in building basements and are sometime contained in metal enclosures. Similarly, in smart agriculture use cases, MTC devices may be located in rural and isolated areas.
3GPP achieves MTC device cost/complexity reduction through various techniques. One technique includes reducing the UE transmit and receive bandwidth from 20 MHz to 1.4 MHz. The reduced bandwidth means that the UE will transmit or receive up to 6 physical resource blocks (PRBs) at 180 kHz instead of up to 100 PRBs.
To achieve adequate coverage for low-complexity UEs and other UEs operating delay tolerant MTC applications, time repetition and retransmission techniques may be used to facilitate energy accumulation of the received signals in both the downlink and uplink. The amount of repetition of the physical signals and channels transmitted to and from a UE may be optimized with respect to the UE's coverage situation (e.g., more repetitions in bad coverage situations than in good coverage situations).
One particular type of information that benefits from reliable transmission is system information. To acquire access to a cell and generally operate within the cell and network, a wireless device acquires system information that is repeatedly broadcasted by the network. The main part of the system information is included in various System Information Blocks (SIBs). Each SIB contains a common type of information (e.g., access parameters in SIB1 and radio resource configuration in SIB2). LTE currently defines seventeen different SIBs.
The various SIBs are mapped to different System Information (SI) messages, which correspond to the transport blocks transmitted over the air interface. An exception is SIB1, which is transmitted without a mapping to an SI message. SIB1 is also special in that it is transmitted using a fixed schedule (periodicity of 80 ms, with repetition in subframe #5 every 20 ms). It contains the SIB-to-SI mapping and scheduling information for the SI messages.
For SIB1 and the SI messages to be decodable by all UEs in a cell, including UEs at the cell edge and UEs with poor radio conditions, the network may use a technique referred to as soft combining. The soft combining technique repeats the transport block containing SIB1 or an SI message over multiple sub frames. The receiver combines the received transmission with previous transmissions and attempts to decode the message. After a sufficient number of repetitions, the accumulated signal energy may be high enough that the decoding succeeds.
Another SIB1, referred to as CE-SIB1, may specifically account for low complexity UEs operating in enhanced coverage. Similarly to legacy SIB1, CE-SIB1 may contain SIB-to-SI mapping and scheduling information for the SI messages. A difference is that CE-SIB1 may be sent within a reduced bandwidth and using more repetitions than legacy SIB1 so that low complexity UEs in enhanced coverage are capable of receiving it. The contents of CE-SIB1 may be the same as the contents of the legacy SIB1 or a subset of the contents of legacy SIB1. CE-SIB1 may also include (a subset of) information from other legacy SIBs (e.g. SIB2, SIB14, SIB3, etc.).
The transport block containing the SI message may or may not be associated with further resource allocation information transmitted in a downlink control information (DCI) over a physical downlink control channel (e.g. EPDCCH). For example, the resource allocation information may be transmitted either in its entirety in CE-SIB1 or partly in CE-SIB1 and partly in a DCI. If the SIB-to-SI mapping information is small enough, it may be transmitted in unused (spare) bits in the master information block (MIB) on the physical broadcast channel (PBCH).
Enhanced coverage may also be supported for so-called normal-complexity UEs (i.e., UEs without the 6 PRB bandwidth restriction or other types of complexity reductions). In one example, the normal-complexity UE may mimic some of the behaviors of a low complexity UE when operating in enhanced coverage. In particular, this means that a normal-complexity UE may read CE-SIB1 and the associated SI messages. The examples and embodiments described herein do not distinguish between a low complexity UE and a normal-complexity UE operating in enhanced coverage, and are applicable to both.
A problem with re-using existing SI message scheduling used with SIB1 with CE-SIB1 is that the existing SI message scheduling does not facilitate differentiation between low complexity UEs in normal coverage and those in enhanced coverage. For example, low complexity UEs in enhanced coverage may attempt to acquire all SI messages listed in CE-SIB1, even those containing non-essential SIBs (i.e., SIBs that are not strictly necessary for low complexity UEs in enhanced coverage). Because a low complexity UE in enhanced coverage may need to accumulate a very large number of repetitions to successfully decode an SI message, this could potentially lead to a large waste of time and power. The lack of differentiation also implies that all SI messages are sent with the same (i.e., maximum) repetition level, which may be spectrally inefficient and result in increased system overhead.